Urodynamics (UDS) is a branch of diagnostic studies devoted to the functional assessment of the lower urinary tract (LUT). Air charged catheters (ACC) are one type of assessment tool used in UDS to quantify pressures acting on the bladder, urethra and abdomen. For the first time, a 5Fr ACC is being developed for UDS studies. Up till now, ACC have only been available in a 7Fr size. A 5Fr catheter may be more suitable in size for some pediatric populations. ACC have less motion artifact than water catheters, important for pediatric patients who are often unable to remain still during UDS [1]. The aim of this study is to assess the potential clinical suitability of the new 5Fr ACC through five common UDS characteristics: pressure linearity, pressure offset, frequency response, pressure drift, and infusion rate [2].

For this study, 184 total T-DOC® 5Fr ACC were used in four tests to characterize the observed parameters. For the first test (n=118), pressure offset and linearity data was obtained by inserting each catheter’s pressure sensing balloon into a custom pressurized chamber, and catheter pressure read by a transducer (Digiquartz® Model 745, Paroscientific Inc, Redmond, WA). The balloon was charged according to the manufacturer’s instructions, and influenced by step pressure changes in the chamber. Offset was calculated by taking the difference between applied pressure and pressure detected by the transducer. Regression analysis was performed to determine the linearity of the detected catheter pressure over increasing chamber pressure. Data for the second test (n=40), frequency response, was collected by placing a charged ACC sensor balloon inside a water column. The column was rigidly fixed to a pressure pulse generator (PPG601A, Flometrics, Carlsbad, CA) that introduced sinusoidal pressure waves with a sweep function of frequencies from 1 to 30Hz. A 50% amplitude attenuation (-6db) frequency was used to identify the functional bandwidth of the catheter frequencies [1].  The third test (n=5), pressure drift, used a custom pressure chamber held at 50 cmH2O, inside a water bath at 37°C, for 2 hours with catheter pressures recorded at t=0, 1 and 2 hours. The fourth test (n=15), infusion rate, was a simulated bladder infusion (Nexam Pro with 5-roller pump, MMS, Enschede, NL), that used a calibrated scale for post-infusion comparison. Each tested rate was infused for 5 minutes to attain a time-averaged infusion volume, except at 5mL/min which was infused for 20 minutes as per acceptability guidelines for low-infusion rates [2]. Each parameter is reported as mean ± standard deviation. Using a two-tailed z-test at a 95% confidence interval, the available data for frequency response and pressure offset were found to have adequate sample sizes. The infusion rate accuracy and pressure drift tests were part of a pilot study and significant conclusions cannot be made about these data sets at this point.

The 5Fr ACC showed linearity of 0.99 ± 0.01, between 136 to 272cmH20, with a r^2 value of 1.00. The offset was shown to be -0.18 ± 1.09%, -1.03 ± 0.83%, -1.65 ± 0.65% at pressures of 136, 204 and 272cmH2O respectively. The catheters demonstrated an increased offset value, but decreased variability, as the catheters reached the maximums of their test range. The frequency response test demonstrated an immediate amplitude decay. The initial output voltage signal was attenuated by 50% at 7.13 ± 0.88Hz. The pressure drift test showed a slow pressure decay over a two-hour period with a reduction in measured pressure of 0.48 ± 1.08% at 1hr and –0.36 ± 1.46% at 2 hr. The infusion rates of 20-70mL/min produced offsets between 0.43% and 1.54% in total volume while at 5mL/min yielded on average -0.14mL/min offset [2]. The mean test results are illustrated in figure 1  with graphs A-D corresponding to tests 1-4 respectively.

A linearity of 0.99 suggests that the pressure read from this catheter very closely follows the applied pressure, but was usually slightly lower than the actual applied pressure. The ICS guidelines for urodynamic equipment suggests that the sum of all pressure offsets should not affect pressure accuracy by more than 3% [2]. Most UDS studies do not run longer than one hour, and UDS pressures seldom hold at or above 200 cmH2O. Using the root sum of squares method, the combined pressure drift and linear offset of this catheter, stated after one hour at a pressure of 203 cmH2O is -1.18%. While the pressure drift test was a pilot study and more data is needed to draw a conclusion, the results obtained indicate acceptable total pressure offset. A frequency response of 7.13Hz adequately captures most events in a UDS study [1]. Previous studies on commercially available ACC deemed to capture adequate clinical data, reported frequencies of 3 and 5 Hz [1][3]. The infusion rates from this study indicate acceptable infusion from 5mL/min to 70mL/min, which should fulfill the needs of most UDS studies [2].

The data used in this study for pressure linearity, pressure offset and frequency response suggest that the T-DOC® 5Fr ACC will be suitable for use in UDS studies. More data is needed to draw significant conclusions about the pressure drift and infusion rate.

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